Electromagnetic delay lines



March 12, 1963 H. s. BENNETT 3,031,439

ELECTROMAGNETIC DELAY LINES Original Filed June 22, 1955 2 Sheets-Sheet1 FIG. l2.

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BY JWQ United States Patent 3,081,439 ELECTROMAGNETIC DELAY LINESHerbert S. Bennett, Deal Park, N.J., assig'nor to Capehart Corporation,a corporation of New York Original application June 22, 1955, 'Ser. No.517,131, now Patent No. 2,892,162, dated June 23, 1959. Divided and thisapplication June 19, 1959, Ser. No. 821,538 Claims. (Cl. 333-31) Thisinvention relates to the art of electromagnetic delay lines andparticularly concerns delay lines of miniature size having delaycharacteristics of about 1.5 microseconds and usable in circuitsoperated'at frequencies of about megacycles per second. This applicationis a divisional patent application based on my pending patentapplication Serial No. 517,131 filed June 22, 1955.

The invention in one form is a delay line having distributed constantsand in another form has both distributed and lumped constants generallyidentified herein as a semi-lumped-constant delay line.

Heretofore satisfactory miniature'delay lines operable in the vicinityof 10 megacycles per second providing delays of the order of 1.5microseconds with good voltage responses have not beengenerallyavailable in spite of an ever increasing need for such circuit membersfor use in high frequency assemblies of electronic equipment.

It is thus one object of the invention to provide anovel miniature delayline including a helical coil and a plurality of capacitors connected tothe coil and disposed within it.

It is a-further object to provide a novel miniature delay line withdistributed constants.

It isa further object to provide a miniature delay line having agenerally cylindrical form with a maximum diameter of about one-halfinch and a length'of 'not more than one and one-half inches, andproviding a circuit time delay of approximately one and one halfmicroseconds at a circuit frequency of ten megacycles per second.

It is a further object to providea miniature delay line having lumpedand distributed-constants.

It is a further object to provide a miniature electromagnetic delay linehaving distributed constants, and including a plurality of concentrictubular capacitors and a coil of rectangular'cross section wound on ahollow core.

t is a further object to provide novel methods and techniques formanufacturing miniature delay lines having both lumped and distributedconstants.

A delay line having lumped constants may be realized according to theinvention by 'providinga slotted hollow cylindrical ferrite core onwhich a coil is wound to provide an inductance. A plurality of miniaturecapacitors with glass or mica dielectrics having capacitances ofapproximately 160 micromicrofarads are disposed within the hollow core.The capacitors have metal foil tabs projecting through the slot in thecore and between individual windings or groups of windings of the coil.The capacitors are arranged in a stack which may include about a hundredcapacitors in a block of about 1.4 inches in length. The tabs may besoldered to the respective turns of the coil, or the tabs and windingmay be compressed to effect electrical contact between windings andcapacitor plates. Each capacitor consists of a pair of foil platessecured to a suitable dielectric plate. The dielectric may be glass,mica, or other appropriate dielectric material. The capacitors may beseparated by plastic films having a thickness of the order of one mil.The films may be of precast material such as polytetra-fiuorethylene orother polyhaloethylene. A suitable plastic film for this purpose isprovided by the Dupont Company of Wilmington, 'Delaware, under the nameTeflon. A film may also be provided by coating one side of eachcapacitor with a suitable dielectric material which hardens on drying.

Such a coating may beformed with an acrylic-lacquer. Whatever materialis used as the separating film should have a high dielectric constant,at least 3.1 at one megacycle and at least 6 at 30 megacycles. The powerfactor should be not more than 0.05 and the film must have goodresistance to water, alkali, acids, oils, greases, and chem- -ic'alfumes. Resistance to high temperatures of the order of 500 F. isalsodesirable if the capacitors are'to be soldered to the coil. The coilmay be wound with enameled wire or a-braided type of wire generallyknown as litz wire. A'suitable coil may be 'wound with five strand, 44gauge litz wire. The tabs of the capacitors may be curved around thecoil windings or left uncoiled. The capacitors each have one foil plategrounded or rather connected to a common lead. They are thus arranged inparallel circuit in the delay line. If the tabs of the capacitors arecurved around the coil windings, the total capacitance of the'stack willbe higher than if the tabs are not so curved even'though a heavypressure'may be applied to compress the tabsand coil turns together.Besides the projecting tabs of those plates which contact the coilturns, the capacitors also have foiliplates with projecting portionswhich are connected in common. These projecting foil portionsare'compressed together and may be covered with a conductingsilver paintor may be soldered to-connect allplates with good electrical contact toa common lead. The capacitor stack may be formed into an integral unitby casting into acylindrical plastic resin'block casing orby coating thestackwith an air drying and hardening resin casing.

In accordance with the invention a distributed constant miniature delayline may be constructed by winding a toroidal coil on a hollowcylindrical ferrite core. Two cylindrical ceramic capacitors aredisposed concentrically, one inside and one outside the ferrite core.The inner and outer capacitors may have barium titanate dielectriccores.

The invention will be better understood from the following descriptiontaken together with the drawing, where- 1n:

FIG. 1 is an oblique view of a delay line of semi-lumped constant typeaccording to the invention.

FIG. 2 shows in elevation a capacitor stack.

-FI G.-3is a sectional view'taken on lines 33 of FIG. 2.

FIG. 4 shows an oblique view of a capacitor assembly block.

FIG. 5 is a side elevational view partly in section of another capacitorassembly block.

FIG. 6 is an end viewo'f the block of FIG. 5.

FIG. 7 is a plan central sectional view of a portion of a delay lineincluding the capacitor block of FIGS. 4 and 5.

'FIG. 8 is a top plan view of a delay line.

FIGS. 9, 10, l3, l4, l5 and 16 show ways of arranging capacitor tabs tocontact coil turns.

FIG. 11 is an isometric view of a coil and core assembly.

FIGS. 12, 1 7, 18 show equivalent circuits for delay lines.

FIG. 19 is an exploded isometric view of componentsot adistributedconstant type of delay line.

. FIG. 20 is a sectional view taken on lines 20-20 'of FIG. 19.

FIG. 21 is an end view of a tubular capacitor employed in the delay'lineof FIG. .19.

FIG. 22 is an isometric view of an' assembled distributed constant delayline with a portion broken away toshow internal features.

10 which is a hollow cylinder having a longitudinal slot S. The core maybe composed of ferrite material. A coil 11 of litz wire is wound on thecore. A stack 12 of capacitors is disposed wholly inside the core.Projecting ends 13 of the negative or ground plates of the capacitorsare connected by a wire 14. A portion of the wire has an enameledcoating 15. The wire of the coil is insulated and consists of braidedstrands 16. The plastic insulator films 17 separate the individualcapacitors. Each film separator is generally T-shaped with a rectangularbody B and a tab T. The over-all size of the film separator is somewhatlarger than that of the correspondingly shaped positive plate 18 of eachcapacitor and the tabs T project up through the slot S and above tabs Tof the positive foil plates 18.

In FIGS. 2 and 3 are shown the stacking arrangement of the capacitor.The negative plate 19 is shown uppermost in FIG. 2. Tab portion 13extends below positive plate 18 and dielectric spacer member 25. Theseparators 17 are disposed between the positive plate 18 of onecapacitor and the negative plate 19 of an adjacent capacitor. A space Dexists between tab T of each separator and the tab T of the positiveplate of the next capacitor. One or more coil turns are lodged in thisspace D when the delay line is fully assembled. A separator film 17 isalso placed at the beginning and end of each stack of capacitors. Thewidth of tabs T should be just wide enough to enter slot S in the core.The slot may be about of an inch in width. Instead of litz wire, singleconductor enamel insulated wire may be used. A coil wound with a solidconductor wire will have a Q somewhat lower than that of a coil woundwith litz wire of substantially equal area.

The stack of capacitors may be assembled as shown in FIG. 4. A silverpaint coating 27 covers the exposed tabs 13 of the negative plates and asuitable lacquer or varnish coating 28 covers the bodies of thecapacitors. The tabs T and T are left uncoated.

In FIGS. and 6 are shown another capacitor stack assembly cast in acylindrical resin block 26 with tabs T and T exposed. The negativeplates 19 are connected by the common wire 14 which may be soldered byjoints 29 to the plate portions 13. The diameter of block 26 should besuch that the stack will fit snugly in circular bore 30 of the core asshown in FIG. 7.

In FIG. 8 is shown the tabs T and T extending through slot S. One turnof winding 11 passes between each pair of tabs. When the tabs andwinding are compressed by opposed forces P, P directed along the slot,electrical contact is effected between tabs T and the turns of the wire.In the region of the slot S it will be necessary to remove theinsulating coating of the wire which can be done by any suitablesolvent. In FIG. 8 the tabs T are unsoldered.

In FIG. 9 is shown an arrangement similar to that of FIG. 8 foreffecting electrical contact between the tabs T of plates 18 and theturns of coil 11 by application of a force to bend the tabs in directionA. The tabs and foil plates are coated on one side with a film F of asuitable insulating lacquer. The total capacitance of the stack israther low when contact is obtained only by applied pressure. In FIG. 10the tabs T are shown soldered to turn portions W of the coil by solderjoints 40. The tabs and coil turns are also compressed so that thesolder and wires will be pressed into the bodies of tabs T. Thisarrangement produces a higher coil Q than if the tabs and coil are notcompressed. The reason for increase in Q may be explained by referenceto FIGS. and 17. FIG. 15 shows the tabs T and T and coil turn portions Win an uncompressed state. FIG. 17 shows a constant K type of circuitconstituted by a delay line in which each turn L of the coil isconnected to a capacitor C. There is a distributed capacitance Cexisting by reason of the extended form of the coil. When the assemblyof FIG. 15 is compressed to the form of FIG. 10 the distributedcapacitance decreases to increase the Q of the coil.

Either litz wire or solid copper wire may be used for the coil turnsdepending on the coil Q required. A No. 30 gauge enameled solid wirewill generally produce the same delay time as five strand No. 44 gaugelitz wires. The soldering of the wires and tabs requires microscopicsoldering techniques, since the available width for the joint may beless than one sixteenth of an inch and the over-all length of the delayline including about a hundred stacked capacitors may be an inch to aninch and a half.

FIG. 11 shows a coil assembly in which winding 11 has a plurality ofinterconnected groups of turns L. The coil 11 terminates at its ends inleads 35, 36. When a capacitor stack 12 is disposed in cavity 30, thecircuit arrangement shown in FIG. 12 will be obtained. Capacitors C areconnected between adjacent turns of adjoining coil groups L. Thenegative plates 19 are connected in common to lead 14.

In FIG. 13 is shown one way of connecting tabs T to coil 11. Each foiltab T is bent around a turn of wire and the turns are pressed together.The wire may be a solid conductor or litz wire. Each turn of the windingcontacts a tab T. In FIG. 14 the tab T engages two strands 16 of a fivestrand insulated litz wire. The tab is bent around strands from whichinsulation I has been removed. Plastic separating films 17 may bedisposed between adjacent litz wires. In FIG. 13 one side of eachpositive plate 18 and its tab T is coated with an insulating film F orthe separating films 17 may be used between the several capacitors. Thepresence of insulated turns 16" in the arrangement of FIG. 14 improvesthe resistance to short circuiting when the turns are strongly pressedtogether to effect complete contact between tabs T and the strands 16.

FIG. 15 shows another assembly scheme in which insulation is removedfrom one side of each insulated turn to expose the wire portion W. Theturns are then strongly pressed together to effect electrical contactbetween the coil turns and tabs T. The exposed wire portions and tabsshould be soldered together by joints 40 to obtain optimum contactbetween the turns and tabs. In FIG. 16 two turns of the coil 11 aredisposed between tabs T and one portion W is soldered to the tab T.

A typical miniature delay line constructed according to the invention inwhich the winding includes close turns of five strand No. 44 gaugeinsulated litz wire on a ferrite core and 100 capacitors each having avalue of 160 micromicrofarads, will be about 1.4 inches long and lessthan one half inch in diameter. It will have approximately the followingparameters at 17 megacycles per second.

Inductance 250 microhenries. Delay 1.8 microseconds. Characteristicimpedance ohms.

The ferrite composing core 10 may be powdered iron in a suitable binderor other magnetic diamagnetic, or paramagnetic material. The core mayeven be made of fiber instead of ferrite for certain applications. Thecoil winding 11 as indicated above may be litz Wire or solid enameledwire. The capacitors may have copper, aluminum or brass foil plates withexposed portions silvered to facilitate soldering. The dielectricsbetween plates may be glass, mica (natural or artificial) or othersuitable material. The separator films 17 may be made of a suitable castfilm such as one of the polytetrahaloethylenes. The binder for forming ablock of capacitors may be a cast resin such as an acrylic resinmaterial. The number of turns for the coil winding may be increased ordecreased as desired depending on the particular parameters of the delayline desired.

One important use of the semi-lumped constant type of delay line asdisclosed herein is as a constant K type of low pass filter as shown inFIGS. 12 and 17. The delay line can also be constructed as an m-derivedfilter circuit as shown in FIG. 18 where inductors 51 and 52 are eachequal in inductance value to one-half of that I of inductor 50.Inductors 51, 52 are connected in series. Inductors 50 and capacitor Care connected between inductors 51, 52 and ground wire 14.

In FIG. 19 are shown the several tubular components of a miniaturedistributed constant type of delay line. A helical coil 55 of Wire iswound lengthwise on the hollow cylindrical core 56 to form solenoid H.The core may be composed of ferrite material and provides a dielectricforthe distributed capacitance of the delay line in addition to servingas a support for the coil. The coil may be wound with insulated copperlitz wire. A suitable type has five strands 16 and is No. 44 gauge. Asingle layer winding is used having about forty-four turns betweenterminal leads 35, 36. The coil is rectangular in cross section asclearly shown in FIG. 24. The coil may be wound with insulated solidcopper wire but at high frequencies copper losses due to skin effectsand other causes become excessive so that the braided litz wire must beused to minimize these losses.

A hollow cylindrical core 60 which may be of vitreous barium titanate isprovided as an inner cylindrical capacitor element M for the delay line.Core 60 has several conductive stripes 61 of powdered silver paint onthe outer surface of core 60. The stripes extend lengthwise and arejoined at one end by a circular ring 62. These stripes 61 are used butthis number of stripes may be increased or decreased. A silver layer 64is disposed on the inner surface of core 60. The lead wire 63 issoldered to the layer 64.

Another core 65 has inner and outer surfaces coated with silver layers67 and 66 respectively deposited by electrolytic deposition, evaporationor other suitable method to form tubular capacitor N. Core 60 fitssnugly in the solenoid H formed by coil 55 and core 56 and the solenoidH fits snugly in the tubular capacitor N.

FIG. 22 shows the completed assembly with members M, H, N one Within theother. It is important that the several members make good contact witheach other by a snug frictional fit. Member H is somewhat longer thanmembers M and N as shown in FIG. 22. Some functions of inner member Mare to increase the total capacitance of the delay line and tocorrect-for Q changes, high frequency energy losses, and leakageinductance. These functions are performed to a greater or lesser degreedepending on the orientation of member M with respect to the othermembers, by the mode of connection to the other members, and by thearrangement of stripes on member, M. The stability of capacitance of thedelay line will be improved if the layer 64 is electrolyticallydeposited to produce a stable, uniform thickness of silver in the core60. The inner member M may also be adjusted to improve pulse response ofthe delay line copper may be used instead of silver for the variouslayers but silver is preferred to minimize reflections in the line. Theequivalent circuits for the delay line depending on external circuitconnections may be represented by the constant K circuit of FIG. 17 onthe derived circuit of FIG. 18.

The foregoing is illustrative of preferred forms of this invention andit will be understood that these preferred forms may bemodified andother forms may be provided within the broad spirit of the invention andthe broad scope of the claims.

What is claimed and desired to protect by Letters Patent of the UnitedStates is:

l. A delay line comprising a plurality of concentrically disposed hollowcylindrical cores nested within each other, a first one of said coreshaving a coil of insulated wire wound lengthwise thereon, the windingsthereof extending along the inner and outer surfaces of said core andacross its ends, a second one of said cores being a tubular capacitorhaving inner and outer surfaces thereof coated with a metal film, thefilm on the said inner surface being in close contact with the coil, anda third one of said cores having an inner surface coated with a metalfilm and an outer surface coated with a metal film in the form ofinterconnected metal stripes, said stripes being in close contact withthe coil.

'2. A delay line according to claim 1, wherein said first core iscomposed of ferrite material, and the second and third cores arecomposed of vitreous barium titanate.

3. A delay line according to claim 2, wherein said coil is composed oflitz wire consisting of copper wire strands and the metal films arecomposed of silver.

4. A miniature electromagnetic delay line, comprising an inner hollowcylindrical core, an outer hollow cylindrical core, and an intermediatehollow cylindrical core, all of said cores being concentric with andnested within each other, the inner core having a conductive layer onits inner surface and a lead being connected to said conductive layer, aplurality of longitudinally extending spaced strips mounted on the outersurface of said inner core, a ring connecting said strips at one endthereof, said strips and ring being composed of conductive material, theintermediate core being wound with a coil of insulated wire extendinglongitudinally thereof, said wire coil extending along the inner andouter surfaces of said intermediate core and across its ends to define'asubstantially rectangular cross-section, said wire-wound intermediatecore being mounted on said longitudinally extending strips, said outercore comprising a tubular capacitor having a conductive layer on itsinner surface and a second conductive layer on its outer surface, andbeing mounted on said wire-Wound intermediate core with said innerconductive surface in contact with said coil, and leaves connected toopposite ends of said coil.

5. A delay line in accordance with claim 4 wherein the intermediate coreis composed of ferrite material and the inner and outer cores arecomposed of vitreous barium titanate, said strips, ring and conductivelayers on the inner and outer cores being composed of metallic silver.

References Cited in the file of this patent UNITED STATES PATENTS845,609 Brown Feb. 26, 1907 2,258,261 Roosenstein Oct. 7, 1941 2,387,783Tawney Oct. 30, 1945 2,467,857 Rubel et al Apr. 19, 1949 2,619,537 KihnNov. 25, 1952 2,768,357 Lyons Oct. 23, 1956 2,781,495 Fredrick Feb. 12,1957 2,897,294 Lipkin July 28, 1959

1. A DELAY LINE COMPRISING A PLURALITY OF CONCENTRICALLY DISPOSED HOLLOWCYLINDRICAL CORES NESTED WITHIN EACH OTHER, A FIRST ONE OF SAID CORESHAVING A COIL OF INSULATED WIRE WOUND LENGTHWISE THEREON, THE WINDINGSTHEREOF EXTENDING ALONG THE INNER AND OUTER SURFACES OF SAID CORE ANDACROSS ITS ENDS, A SECOND ONE OF SAID CORES BEING A TUBULAR CAPACITORHAVING INNER AND OUTER SURFACES THEREOF COATED WITH A METAL FILM, THEFILM ON THE SAID INNER SURFACE BEING IN CLOSE CONTACT WITH THE COIL, ANDA THIRD ONE OF SAID CORES HAVING AN INNER SURFACE COATED WITH A